Automatic Power Reduction (APR) is a phrase that is used in the context of laser safety. System fibers frequently have more power than is acceptable for an open interface. Thus, the system must detect an open connection and then lower the power within a certain amount of time to limit exposure and potential eye damage. This is mandated by IEC60825, for example.
Colorless-Directionless (CD). Colorless-Directionless-Add (CDA), and Colorless-Directionless-Contentionless (CDC) Reconfigurable Optical Add-Drop Multiplexer (ROADM) architectures continue to grow in popularity. These ROADM types typically utilize Multi-Fiber Push-On (MPO) style cables or the like to keep the intra-node fibering manageable. Roughly speaking, these multi-fiber cables are treated as a single point source from a laser safety perspective. As such, the sum of the emitted power from all sub-fibers cannot exceed the IEC 60825 Class 1M (“1M”) limit of ˜21.3 dBm based on certain conditions, for example. Therefore, if there are four active sub-fibers per cable, this translates into a per sub-fiber maximum of ˜15.3 dBm.
In previous incarnations of ROADM design, it was possible to conform to this limit without requiring any sort of reactive mechanism. The number of channels, their spectral occupancies, and the system Power Spectral Density (PSD) targets resulted in a total power lower than the per sub-fiber maximum required to be 1M. In general, the PSD targets of channels remains constant regardless of their spectral occupancies. As such, the power requirement for a Colorless Channel Mux/Demux (CCMD) scales not only with the with the number of channels, but also with their spectral occupancies.
The drive for increased capacity, in combination with evolving technology, is resulting in changes that effectively increase the total spectrum, and, correspondingly, power, that a Wavelength Selective Switch (WSS) must direct towards a given CCMD (i.e., per sub-fiber). Increases in baud rate (e.g., 56, 75, 90+GBaud) lead to higher spectral occupancies of those channels. Further, the number of add/drop channels per CCMD is increasing (e.g., 12 to 24+). Still further, channel pre-combining acts as a port multiplier for the number of channels per CCMD (e.g., 4×24).
Together, these changes lead to the potential for a given CCMD to include 96 channels at more than 90 GHz of spectrum each. In effect, this is enough to fill a C or L-band of multiple degrees. As such, it is no longer possible to remain below the 1M limit for cables without an active response. The special challenge is to avoid collateral damage to the signal on other ports. Thus, what is needed is a spectrally selective response to one or more corresponding APR triggers.
In the past, clamp mechanisms or back reflection-based APR were frequently relied upon to maintain laser safety requirements. Clamping amplifiers to a power below the eye-safe limit is no longer possible as a result of the amount of spectrum, and thus power, being routed to a given CCMD port. Back reflection based APR is no longer possible due to the Angled Physical Contact (APC) termination of single-mode MPO cables.